ROVIDED. Endothelial cell (EC) barrier dysfunction, a prominent feature of acute lung injury (ALT), is tightly linked to cytoskeletal remodeling, which leads to the disruption of cell-cell contacts and includes activation of contraction initiated by myosin light chain (MLC)phosphorylation followed by F-actin stress fiber formation and formation of paracellular gaps. Little is known about processes which determine barrier enhancement or protection;however, our published data implicate a critical role for cytoskeletal dynamics in this response. Extracellular ATP is an important vascular mediator, which elicits cellular effects on EC mainly through P2Y family receptors coupled to specific trimeric G- proteins. Our novel findings indicate that ATP at physiologically relevant concentrations produces rapid, sustained and dose-dependent increases in transendothelial electrical resistance (TER), indicating profound barrier enhancement and potently reversed barrier dysfunction elicited by the edemagenic agent, thrombin. Specific depletion of a subunits of Gq and Gi2 significantly attenuated ATP-induced increase in TER indicating the involvement of these G-proteins inATP- induced EC barrier enhancement. The ATP-induced increase in TER is tightly linked to an increase in myosin-associated phosphatase (PPase) 1 (MLCP) activity. Inhibition of PPase 1 abolished the ATP-induced increase in TER and lead to phosphorylation of several cytoskeletal targets includingMLC, ezrin/radixin/moezin (ERM) and caldesmon suggesting that dephosphorylation of these proteins may be involved in the barrier-enhancing effect of ATP. In addition, protein kinase A (PKA) inhibition attenuates both ATP-induced increases in TER and phosphorylation of vasolidator- stimulated protein (VASP), which in the phosphorylated form inhibits stress fiber formation supporting the involvement of the PKA/VASP pathway in ATP-induced EC barrier enhancement. Our working hypothesis is that ATP-induced EC barrier enhancement and cytoskeletal remodeling is dependent, at least in part, upon activation of specific P2Y/G protein complexes followed by coordinated activation of MLCP and PKA signaling. SA#1will define the role of specific P2Y/G-protein complexes in the activation of MLCP- and PKA-dependent signaling. SA#2 will define the involvement of MLCP and its cytoskeletal targets in ATP-induced EC barrier enhancement. SA #3 will explore the molecular mechanisms by which PKA activity is involved in ATP-induced EC barrier enhancement focusing on VASP and MLCP as potential PKA targets. SA#4 will characterize the potential barrier-protective effects of ATP in murine models of ALL These studies will provide an understanding of the novel signaling pathways involved in ATP-induced lung EC barrier enhancement and promise new directions and targets for treatment of lung disorders.